The goal of this proposal is to more fully understand the role of the Gag proteins in the assembly of HIV-1 virions, with an emphasis on understanding the biochemical basis of infectivity-inactivating amino acid substitutions. In particular, the primary sequence requirements for Gag multimerization will be determined, studying both HIV-1 and also other members of the immunodeficiency virus family. The role of viral RNA in Gag multimerization will also be studied. Finally, a selection of techniques will be used to assess how mutations affecting Gag multimerization impact upon HIV-1 infectivity in vitro.